1. Field of the Invention
The invention relates generally to global navigation satellite system (GNSS) receivers having fast times to first fix and particularly to a GNSS receiver that uses a calibrated real time clock (RTC) time for determining a first fix of GPS position and time.
2. Background
The United States government maintains a global positioning system (GPS) having a constellation of earth orbiting GPS satellites. The satellites broadcast GPS signals having location-determination information that can be received and decoded in a GPS receiver for determining a GPS time and a GPS-based geographical position of the receiver.
The acquisition process for finding signal power in a GPS signal involves correlating a pseudorandom noise (PRN) code from incoming satellite signals against locally generated PRN code replicas at incremental time offsets or phase shifts with respect to a local GPS reference clock. The code replicas are phase shifted through an entire code epoch until the phase shifts are found that provide the highest correlations. This process is known as a code search. A raw pseudorange is measured as the time offset between the time-of-arrival of the PRN code received in the GPS satellite signal and the locally generated replica of the PRN code with respect to the local reference clock. The prefix “pseudo” is used to indicate that the range that has an ambiguity due to the time ambiguity of the local reference clock. The GPS receiver then determines data bit timing from inversions of the correlations. The data bit timing, the raw pseudorange and an integer number of code epochs are used in the determination of the total pseudoranges between the receiver and the source of the GPS satellite signal.
When signal power and data bit timing is found, the GPS receiver monitors the data bits until Z-counts are decoded. A GPS time-of-transmission is read from the Z-count for each GPS satellite. The GPS times-of-transmission are used with ephemeris information that is available in the GPS signal data bits or stored locally and updated at intervals for calculating the present locations-in-space of several GPS satellites. Having the locations-in-space and pseudoranges for four or more GPS satellites, the GPS receiver solves simultaneous equations to correct the time ambiguity in the local GPS reference clock and resolve the three dimensions of the geographical position of the GPS receiver.
In a first problem, in some places there is not enough energy available in the GPS signal for signal acquisition by correlation over an integration time period that is limited to a single code epoch. A GPS receiver could perhaps overcome this problem by accumulating correlations over integration time periods greater than one code epoch. However, this is difficult because the correlations are inverted by changes in senses of the GPS data bits. Workers have proposed storing the expected GPS data bits in order to calibrate for the correlation inversions. However, in order to use the expected data bits, the transition times are needed. This leaves a standoff where the multi-epoch integration time period that is required for signal acquisition in order to read the accurate GPS time requires an accurate timing of the bit transitions, but the accurate timing of the bit transitions is not available until signal acquisition has been obtained using the multi-epoch integration period.
In a second problem, for many market applications GPS receivers need a fast time to a first fix (TTFF) of GPS position and time. One requirement in order to have a fast TTFF is the availability of the ephemeris orbital parameters in order to quickly compute the locations-in-space of the GPS satellites. Existing GPS receivers meet this requirement by storing ephemeris parameters from a recent fix or receiving the parameters in a radio signal. However, even when ephemeris parameters are available in the GPS receiver, an accurate local knowledge of time is required to use the ephemeris parameters for computing satellite locations-in-space. This time is normally received in the data bits of the GPS signal Z-count. Unfortunately, this adds time to the time to first fix because the Z-count repeats only every six seconds. Further, two passages of the Z-count may be necessary in order to verify that the Z-count is not being mimicked by other data bits.
Therefore, existing GPS receivers have had difficulty for low level signal acquisition and fast TTFF because an accurate time is not available in the GPS receiver before the signal is acquired and before a Z-count is decoded.
Existing GPS receivers have devices called real time clocks that are distinguished from other clocks in the GPS receiver, such as the microprocessor clock, the signal and navigation processor clocks and the internal GPS reference clock, by having their own resonant device and having extremely low power consumption. Real time clocks typically operate with a separate small battery to keep track of an approximate time even when the GPS receiver has power off. Existing GPS receivers commonly use time from the real time clock at power turn on in order to predict which GPS satellites are available in the sky and to estimate Doppler frequency shifts. It might at first be thought that such real time clock could be also used to provide the accurate time to assist the GPS receiver for signal acquisition at low signal levels by obviating the need to decode the Z-count. Unfortunately, existing real time clocks having sufficiently low power consumption to be allowed to be always-on do not have the time resolution that is required and have time drift rates that are too great to provide the time accuracy that is needed.
Three techniques have been used to provide accurate local times in GPS receivers at power turn on. A first technique uses an atomic clock. However, an atomic clock may more than double the cost of a modern small GPS receiver. A second technique uses a radio signal transfer time standard. However, the use of such radio signals adds a great deal of complexity of the GPS receiver and the signals are not always available. Further, the radio receiver may be more expensive than the GPS receiver. In a third technique, the local GPS reference clock that is required in the GPS receiver for correlations is used always-on in order to provide an accurate time at turn on. This technique requires that the local GPS reference clock is stabilized for long term accuracy and continues to be powered when the GPS receiver is off or in standby. The U.S. Pat. No. 5,629,708 “GPS Receiver Having an Initial Adjustment for Correcting for Drift in Reference Frequency” by Rodal et al and U.S. Pat. No. 5,854,605 “GPS Receiver Using Data Bit Timing to Achieve a Fast Time to First Fix” by Gildea, both incorporated herein by reference, exemplify this technique. However, the power consumption of the circuitry for the local GPS reference clock is sometimes too high to be always-on in a modern low power GPS receiver.
There is a need for an improved way for a GPS receiver to estimate an accurate local time at turn on for assisting signal acquisition and avoiding the need to receive a Z-count.